The Northern Ireland Civil Rights Movement between the 1960s and early 1970s shifted from an inclusive, reformist movement to an exclusivist, ethnonationalist one. What is the explanation for such a significant transformation? This chapter seeks to answer the question by looking at the complex interactions of political opportunities/threats and the internal dynamics and competitiveness between different organizations and groups within the movement. What I am suggesting in this work is that much of the process of social movement development is understandable only by looking at the broader political environment as well as by looking within the movement itself. Seeing social movements as heterogeneous networks that develop interactively over time and through different stages of mobilization, I will then show how shifting political opportunities influence the process of social movement development and how this process is led by the congruence of those mobilizing messages, which best align with the dominant representation of the political environment present at a given stage.